Low thermal coefficient shadow masks with resilient supports for use in color picture tubes

ABSTRACT

In a shadow mask assembly for use in a color picture tube of the type comprising a shadow mask, a supporting frame for supporting the shadow mask, and a plurality of supporting members for mounting the shadow mask on the supporting frame, the shadow mask is made of a material having a small coefficient of thermal expansion such as invar, the supporting frame is made of a material having a larger coefficient of thermal expansion such as soft steel, and the supporting members are constructed to absorb the difference in the thermal expansion of the shadow mask and the supporting frame.

United States Patent [191 Kawamura et al.

[4 1 Apr. 30, 1974 [75] Inventors: Takao Kawamura; Akio Yamaguchi;

Kenichi Fukuzawa, all of Mobara, Japan [73] Assignee: Hitachi, Ltd., Tokyo, Japan [22] Filed: Nov. 7, 1972 [21] Appl. No.: 304,510

[30] Foreign Application Priority Data H1956 Grimm et al. 313/92 B 2/1968 Demmy 313/85 S Primary Examiner-Archie R. Borchelt Assistant Examiner-Wm. H. Punter Attorney, Agent, or FirmChittick, Thompson &

Pfund [57] ABSTRACT In a shadow mask assembly for use in a color picture tube of the type comprising a shadow mask, a supporting frame for supporting the shadow mask, and a plurality of supporting members for mounting the shadow mask on the supporting frame, the shadow mask is made of a material having a small coefficient of thermal expansion such as invar, the supporting frame is made of a material having a larger coefficient of thermal expansion such as soft steel, and the supporting members are constructed to absorb the difference in the thermal expansion of the shadow mask and the supporting frame.

10 Claims, 5 Drawing Figures rmuiimen 30 m4 SHEET 1 OF 2 FIGJ PATENTEDAPR 30 1914 SHEET? [1F 2 LOW THERMAL COEFFICIENT SHADOW MASKS WITH RESILIENT SUPPORTS FOR USE IN COLOR PICTURE TUBES BACKGROUND OF THE INVENTION This invention relates to the improvement of a shadow mask assembly for use in a color picture tube.

In the operation of a color picture tube including a color selection electrode in the form of a shadow mask provided with a plurality of small perforations for transmitting an electron beam, the temperature of local portions of the shadow mask rises thus causing thermal expansion thereof due to'the collision of the electron beam. After an extended period of-operation, the assembly of the shadow mask and its supporting frame undergoes thermal expansion with the result that the positions of the perforations of the shadow mask are displaced from their normal positions thus causing a socalled landing error phenomenon wherein the electron beam is caused to impinge upon a point other than a prescribed phosphor dot. Various proposals have been made to solve this problem.

According to one approach, leaf springs mounted on the supporting frame are constructed to have a characteristic of a bimetal so as to displace the shadow mask toward the fluorescent screen as the temperature rises for preventing degradation of the color purity.

Recently, however, the requirement for the operating condition of a color picture tube became severe. Thus, for example, it is the present day tendency to operate the television receiving set with reproduced pictures at a higher contrast and brighter image over a long period. When a picture of higher contrast and brighter image is displayed on the fluorescent screen, an electron beam of high density of electrons impinges on the bright portion of the picture thus rapidly generating a large local heat. However, since the shadow mask is generally made of a thin sheet of soft sheel having an extremely small heat conduction, most of the generated heat must be dissipated by radiation. For this reason, above described local rapid temperature rise results in a local expansion due to small heat dissipation. Conseof the shadow mask or for the entire shadow mask assembly. However, such a material should have a coefficient of thermal expansion of less than about 80 X 10 (at a temperature of from 20 to 350C). Usually an iron-nickel alloy or invar is used for this purpose. However, such is material is considerably expensive so that when it is used for the supporting frame which is massive, the cost of the shadow mask assembly is greatly increased.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide an improved shadow mask assembly of simple construction and low cost but capable of preventing the degradation of the color purity or colorftone of the reproduced picture due to a local or entire expansion of the shadow mask caused by the temperature rise thereof.

Another object of this invention is to provide a new and improved shadow mask assembly wherein the supporting members for mounting the shadow mask on the supporting frame are formed integral with the shadow mask thus facilitating the fabrication of the shadow mask assembly.

According to this invention, there is provided a shadow mask assembly for use in a color picture tube of the type comprising a shadow mask, a supporting frame for supporting the shadow mask and a plurality of supporting members for mounting the shadow mask on the supporting mask, wherein the shadow mask is made of a material having a small coefficient of thermal expansion, the supporting frame is made of a material having a larger coefficient of thermal expansion, and the supporting members are constructed to absorb the increment difference between the shadow mask and the supporting frame due to the thermal expansion.

BRIEF DESCRIPTION OF THE DRAWING The invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which:

FIGS. 1, 2 and 3 are side views, partly broken away, of shadow mask assemblies, each embodying the invention; I

FIG. 4 is a partial side view of a modified embodiment of this invention and FIG. 5 shows a plan view of ashadow mask assembly embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 of the accompanying drawing shows a side view of a shadow mask assembly 1 of this invention before it is 'operated and FIG. 2 shows the same mask after its temperature has increased during operation. The shadow mask 2 is made of a material of small coefficient of thermal expansion, for example a Fe Ni alloy containing about .35 to 50% of nickel. The supporting frame 3 occupying the substantial percentage of the weight of the assembly is made of an ordinary material such as soft steel, for example. The shadow mask 2 is mounted on the supporting frame 3 by means of a plurality of supporting members 4 each with one end welded to the inner surface of the skirt of the shadow mask and the other end welded to the supporting frame as at 6. Each supporting member 4 is constructed to have such configuration that absorbs the increment difference between the shadow mask 2 and the supporting frame 3 due to the thermal expansion, the members 4 being formed as resilient tongues e. g., metal strips such as lnvar. With this construction, since the shadow mask is entirely made of a material having a small coefficient of thermal expansion, temperature rise of a portion or entire portion of the shadow mask caused by the collision of the electron beam would not cause any appreciable thermal expansion of the shadow mask. On the other hand, inasmuch as the supporting frame 3 is made of an ordinary material having a substantially large coefficient of thermal expansion, a soft steel, for example, its thermal expansion is large so that the supporting frame 3 expands as shown in FIG. 2 when it is heated. According to this invention, however, since the supporting members 4 interconnecting the shadow mask and the supporting frame are constructed to absorb the increment difference between these two members due to the thermal expansion, the thermal expansion of the supporting frame 3 does not affect in any way the shadow mask thus efficiently preventing displacement of the fine perforations of the shadow mask.

For example, the effective length h of the supporting members 4 of a 20 inch color picture tube was made to be more than 6 mm. The shadow mask 2 was made of invar (a Fe Ni alloy containing about 36 to 43 percent of nickel) having a small coefficient of thermal expansion of the order of 40 60 X (at 20 to 350C). With this construction, when the temperature of the shadow mask was increased to the mean operating temperature of the color picture tube which is about 30C higher than room temperature, the maximum displacement of the perforations of the shadow mask in the direction of arrow B shown in FIG. 2 was only one micron which is of course negligibly small.

FIG. 3 shows a modified embodiment of this invention wherein supporting members 7 in the form of tongues are formed in the peripheral skirt 6 of the shadow mask 2 made of a similar material as the first embodiment. The ends of the supporting members 7 are welded at points 6 to the supporting frame 3 of soft iron. Grooves 8 are formed on both sides of each supporting member 7 so as to permit it to absorb the increment difference between the shadow mask 2 and the supporting frame 3 due to the thermal expansion. If the circumstances permit, the supporting members 7 may be formed on the lower edge of the peripheral skirt 6 of the shadow mask 2 as shown in FIG. 4.

Where the embodiment shown in FIG. 4 is applied to v a 20 inch color picture tube, the object of this invention can be accomplished by designing the supporting member 7 to have a width of about 10 to mm, an effective length h of about 10 to 30 mm, with eight to 16 such members used to interconnect the mask 2 and frame 3. During the fabrication of the color picture tube, as the shadow mask assembly is heated to a maximum temperature of about 500C, it is necessary to provide a gap between the shadow mask and the supporting frame sufficient to absorb the increment difference due to the thermal expansion at such a high temperature. In this embodiment, where the shadow mask is made of invar containing about 36 percent of nickel and having a coefficient of thermal expansion of about 80 X 10 at 500C a minimum gap of about 0.4 mm is required to exist between the shadow mask and the supporting frame.

For a shadow mask assembly of a color picture tube having a rectangular fluorescent screen it is advantageous to position the supporting members in the following manner. More particularly, as shown in FIG. 5 which shows the bottom plan view of the shadow mask assembly, supporting members 4a through 4h (7a through 7h) are positioned at the centers A, to A of respective sides of the rectangle and at all or most of the corners B through 8,. With this arrangement, since respective pairs of the supporting members 4a (7a) and 4b (7b); 40 (7c) and 4d (7d); 4e (7e) and 4f(7f); and 4g (7g) and 4h (7h) are symmetrical with respect to the center of the picture, these supporting members cooperate to absorb any increment difference between the shadow mask and the supporting frame due to the thermal expansion, thus preventing any displacement of the shadow mask in the direction of its plane, as well as the distortion thereof.

While in FIG. 1, the supporting members are provided one at one point, two or more of such supporting members may also be provided closely adjacent to each point. However, where two or more such members are located closely adjacent along each side, it is necessary to construct them so as to absorb also the increment difference due to the thermal expansion in the peripheral direction.

Although in the foregoing embodiments, the configuration of each supporting member was shown as an elongated piece, any configuration may be used for the supporting member so long as it can absorb the increment difference between the shadow mask and the supporting mask due to the thermal expansion and can mechanically interconnect these members, such as a bent resilient member.

Furthermore, in the above described embodiments, both the shadow mask and the supporting frame were provided with peripheral skirt it will be clear that such skirt may be provided on either one of these members.

What is claimed is:

1. In a shadow mask assembly for use in a color picture tube of the type comprising a shadow mask, a supporting frame for supporting said shadow mask, and a plurality of supporting members for mounting said shadow mask on said supporting frame, the improvement wherein said shadow mask has a peripheral skirt and is made of a material having a small coefficient of thermal expansion, said supporting frame is positioned within said skirt and has a peripheral edge spaced a predetermined distance from the inner wall of said skirt, said frame being made of a material having a larger coefficient of thermal expansion than said shadow mask, and said supporting members are constructed as resilient members connected at opposite ends to said edge and said inner wall to absorb the difference in the thermal expansion of said shadow mask and said supporting frame during normal operation, said predetermined distance preventing distortion of said shadow mask at elevated temperatures during fabrication of said tube.

2. The shadow mask assembly according to claim 1 wherein said shadow mask is made of a Fe Ni alloy having a coefficient of thermal expansion of about 40 to X 10" at a temperature of 20 to 500C and said supporting member is made of soft steel.

3. The shadow mask assembly according to claim 1 wherein one end of each of said supporting members is welded to the skirtof said shadow mask and the other end is welded to said supporting frame.

4. The shadow mask assembly according to claim 1 wherein said supporting members are tongues integrally formed with the peripheral skirt of said shadow mask and the free ends of said tongues are welded to said supporting frame.

5. The shadow mask assembly according to claim 1 wherein some of supporting members are arranged symmetrically with respect to the center of said shadow mask.

6. The shadow mask assembly according to claim 1 wherein each supporting member is formed in the pe- 9. The shadow mask assembly according to claim 5 wherein said shadow mask is generally rectangular and said supporting members are located with at least one at four comers of the rectangle.

10. The shadow mask assembly according to claim 5 wherein said shadow mask is generally rectangular and said supporting members are located with at least one at the center of four sides and at least one of four corners of the rectangle. 

1. In a shadow mask assembly for use in a color picture tube of the type comprising a shadow mask, a supporting frame for supporting said shadow mask, and a plurality of supporting members for mounting said shadow mask on said supporting frame, the improvement wherein said shadow mask has a peripheral skirt and is made of a material having a small coefficient of thermal expansion, said supporting frame is positioned within said skirt and has a peripheral edge spaced a predetermined distance from the inner wall of said skirt, said frame being made of a material having a larger coefficient of thermal expansion than said shadow mask, and said supporting members are constructed as resilient members connected at opposite ends to said edge and said inner wall to absorb the difference in the thermal expansion of said shadow mask and said supporting frame during normal operation, said predetermined distance preventing distortion of said shadow mask at elevated temperatures during fabrication of said tube.
 2. The shadow mask assembly according to claim 1 wherein said shadow mask is made of a Fe - Ni alloy having a coefficient of thermal expansion of about 40 to 80 X 10 7 at a temperature of 20* to 500*C and said supporting member is made of soft steel.
 3. The shadow mask assembly according to claim 1 wherein one end of each of said supporting members is welded to the skirt of said shadow mask and the other end is welded to said supporting frame.
 4. The shadow mask assembly according to claim 1 wherein said supporting members are tongues integrally formed with the peripheral skirt of said shadow mask and the free ends of said tongues are welded to said supporting frame.
 5. The shadow mask assembly according to claim 1 wherein some of supporting members are arranged symmetrically with respect to the center of said shadow mask.
 6. The shadow mask assembly according to claim 1 wherein each supporting member is formed in the peripheral skirt of said shadow mask by cutting grooves on both sides of said supporting member.
 7. The shadow mask assembly according to claim 1 wherein said supporting members are projecting tongues formed on the outer edge of the peripheral skirt of said shadow mask.
 8. The shadow mask assembly according to claim 5 wherein said shadow mask is generally rectangular and said supporting members are located with at least one at the center of four sides of the rectangle.
 9. The shadow mask assembly according to claim 5 wherein said shadow mask is generally rectangular and said supporting members are located with at least one at four corners of the rectangle.
 10. The shadow mask assembly according to claim 5 wherein said shadow mask is generally rectangular and said supporting members are located with at least one at the center of four sides and at least one of four corners of the rectangle. 